1. Field of the Invention
The present invention relates generally to motors, and more specifically to piezoelectric and other field actuator motors employing motive power elements which physically elongate, bend or otherwise change dimensions responsive to changes in electrical or magnetic fields.
2. State of the Art
Traditional electric motors cause a shaft or rotor to rotate by creating a magnetic field between a primary winding in a stationary portion, or stator, of the motor and a secondary winding associated with the shaft or rotor. Such motors are relatively large and heavy relative to output torque. Such motors, for many applications, must be connected to transmission systems that alter speed and torque output and, in some instances, convert the rotary movement of the motor rotor to linear movement. These transmission systems, however, add substantially to the size, weight and complexity of the motors. Altering the electrical power input to such motors also provides some adjustability of output, but in most instances such adjustability is limited in range and, as with transmission systems, adds bulk, complexity and cost to the motor system.
Various other types of electric motors have been developed that employ piezoelectric, magnetostrictive, or electrostrictive actuators as motive power elements, rather than magnetic attraction or repulsion as in traditional electric motors. A piezoelectric actuator has a first length when a first voltage (or electric field) is applied across it and a second length when a second voltage is applied across it. An electrostrictive actuator has a first length when a first voltage is applied across it and a second length when a second voltage is applied across it. A magnetostrictive actuator has a first length when a first magnetic field is applied to it and a second length when a second magnetic field is applied to it. As used herein, a term "field actuator" may refer to a piezoelectric, magnetostrictive, or electrostrictive actuator.
It is also contemplated that piezoelectric and other field actuators configured as "bending" actuators in structures which behave similar to bimetallic strips employed in thermostats are also encompassed by the term "field actuator," as are shape memory alloy structures exhibiting similar dimensional variances in response to temperature fluctuations. Therefore, it may also be suitable to characterize the term "field actuator" as encompassing structures adapted to vary in at least one dimension responsive to application or removal of any energy field.
Motors employing piezoelectric actuators as motive power elements have been used in the prior art to create linear and rotary movement. For example, U.S. Pat. No. 5,027,027 to Orbach et al. describes a linear motor referred to as an "Inchworm" motor that includes forward, center, and rear piezoelectrically activated cylindrical elements arranged about a shaft. The shaft is moved forward, for example, by clamping the forward element, extending the center element, clamping the rear element, and releasing the forward element.
U.S. Pat. No. 4,578,607 to Tojo et al. describes a system in which piezoelectric actuators move sections to rotate a disk. The disk is lowered onto the sections after which they are moved by the actuators. The disk is then raised while the actuators reset. Some actuators elongate during the time other actuators contract.